The present invention relates to a substrate composed of at least one cycloolefin copolymer for recording media and to a process for producing it.
To produce recording media, such as optical information carriers, for example, optical disks or compact disks, various layers composed of nitrides, oxides, rare earth/transition-metal alloys are sputtered onto a prestamped substrate composed of polycarbonate resin, polymethylmethacrylate, epoxy resin, polysulfone, polyether sulfone or polyether imide. The reproduction accuracy of the recorded information and the long-term stability of the information carrier are strongly material-dependent under these circumstances; the thermal dimensional stability and the birefringence of the substrate material, for example, affect the reproduction accuracy in a decisive way, while the moisture absorption of the plastic material affects the long-term stability of the physical properties of the recording layers quite substantially. The substrates are produced by injection-molding technology, the groove or pit matrix being transferred to the plastic substrates by means of an original (stamper). The fidelity of this copy to the master depends very strongly on the processability of the injection-molding material. It is known that polymeric materials which have a good flowability under the processing conditions also ensure a good imaging quality.
EP-A 0 310 680 describes a recording medium in the form of a magnetooptical storage disk (MOD) having a substrate composed of an amorphous ethylenetetracyclododecene copolymer which supports a magnetooptical recording layer composed of a quaternary, amorphous rare-earth/transition-metal alloy composed of Tb, Fe and Co, with Pt or Pd as further alloying components.
EP-A 0 387 016 discloses in Example 4 a magneto-optical recording layer composed of Te, Ge and Cr which has been sputtered onto a substrate composed of an amorphous copolymer of ethylene with 1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene (designated DMON for short).
Other known substrate materials are polymer blends of polycarbonate and polystyrene and cycloolefin copolymers, such as are described in U.S. Pat. No. 4,614,778 and in EP-A 0 387 018, column 5, lines 5 to 24.
The structure of a storage disk (OD) for which the substrates of the invention are suitable is described in German Patent Application P 41 37 427, corresponding to U.S. application Ser. No. 07/974,856, the disclosure of which is hereby incorporated by reference.
The reproduction accuracy of the recorded information and the long-term stability are significantly affected by the plastic substrate used, that is to say by the processability of the raw material and the thermal stability of the substrate. The transfer of the groove matrix of the stamper to the substrate during the injection-molding operation is worth improving in the case of the plastic substrates used, which are composed of PC, PMMA and the hitherto known COC (cycloolefin(co)polymers). Inter alia, the rejection rate in the disk production could be reduced, accompanied by simultaneous increase in the quality of the structure transferred by the stamper matrix.